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All last week, since Monday has been an insane heat wave in Connecticut. Starting hot on Monday each consecutive day became hotter, with an average mean temperature in my backyard reading at 115 degrees. Original readings of pond were 7.0 Ph, and Zero across the board for Ammonia, nitrite and nitrate. This pond, set up in April, is -not-cycled, all attempts to cycle it, from adding in material to the filters from an established tank in my home to letting nature take its course, have failed. The pond is monitored closely due to this.

We left Friday night for the weekend, to return Sunday ((only gone a day and a half)), with our usual care giver for cats and fish. Saturday was INSANELY hot, nearing 119 registered on our back yard weather station. The heat "broke" with a 3 minute shower, but then rose again, humid, sticky and disgusting with evening temps in the mid 80"s. Our fish/pet caregiver had inspected fish Sunday morning, and all was well.

We returned Sunday mid afternoon at 3pm..... to find the pond an absolute bog. The SMELL was not to be believed. Our large butterfly koi was dead, floating, and when netting it out seemed to partially disintegrate, leaving an oily slime and residue of its melting body all over the surface. Turning the pumps off to get a clear look at what the heck was going on, our other Koi was dead, stuck to the filter intake, and had the same disintegrating quality. The other fish were clustered all in one corner, barely moving, and bearing white streaking, white mouths, and white clouded eyes.

Water testing showed a total Ph crash, and ammonia of 2.0ppm. Again, the smell, there are no words in the english language to describe it.

Completed an immediate 75% water change, and rinse out of filter media in pond water (removed to a bucket for such). Ammonia taken down to .5ppm, but Ph remained crashed. Fish perked up, but not terribly much.

So far, have added as of this morning Seachem neutral regulator. I am NOT a fan of chemically altering Ph, but in this case I made the exception. Trying to calculate crushed coral for this volume of water would be a nightmare. I have also added Kordon Rid Fungus, as it is what appears to be on the fishes body and eyes.

I have no idea what just happened here. The fish, and Koi were completely fine and marking free 11 a.m. Sunday morning as assessed by our caretaker who's had years of experience dealing with our aquariums and pond. Friday when we had left, they were all in good health, and the pond was absolutely crystal clear.

I desperately want to save the fish I have remaining, but they are in some pretty bad shape. Any input on this matter would be highly appreciated.

That really sucks, s0rry t0 hear it! It has been the Dickens here in Ct hasn't it? Terrible h0t, even at night.
t0 me, unfortunately, that in 4 h0urs - I find it hard t0 believe they w0uld be that far g0ne- n0t imp00ssible- but the caretaker might have stayed in due t0 the heat and said he checked- if the fish were falling apart when y0u g0t h0me it sounds like they were dead a little longer than 4 h0urs. Terrible hit t0 take, I wish I c0uld 0ffer ya s0mething 0ther than c0nd0lences. Maybe if y0u dug a channel and extended y0ur plumbing bel0w the fr0st line ( appx 6ft here in New England) that y0u can use a geothermal based the0ry t0 rid the water 0f s0me excess heat by using the earth as a radiat0r? and it w0uld keep the plumbing fr0m freezing up in the c0ld as well- earth temp at 6 feet deep is in the higher 50's.

Fr0m what I read real quick in a c0uple 0ther f0rum p0sts fr0m p0nd f0rums, Hostas are fine with fish, clearly they are terrestrial plants and wouldn't be planted IN the p0nd, but the leaves falling in shouldn't be a w0rry apparently.

I have approximately the same size pond with a lot more stock and live in PA. We didn't have quite the blast of heat you did (mid 90's) but all fish were fine.

I may have missed this but do you have any floating plants in the pond? Mine is out in full sun so I keep water lettuce & hyacinths to provide shade... so muck so it's hard to see the fish!

I would start to think possible chemical poisoning? Do you or any neighbors treat their lawns? Could it have been a windy day and there was over spray from a treatment? I feel so bad for you and hope the issue gets resolved.

Thanks. The pond is well above ground, so its not likely run-off chemicals getting in. It has been windy, but far as I know no neighbors have done anything. I only have lily and lily pads in there, not enough, and cat tails. I will look into getting water lettuces, but not sure locally where i could get those.

We performed a 50% water change day before yesterday, and though the water is cloudy, and still has a faint whiff of "dead fish bog", I am surprised that all the remaining fish seemed to have bounced back in 24 hours. I treated water with Prime, as well as a dose of Kordon Kill Fungus, though with such rapid recovery I sincerely doubt that they had fungus at all. I am now suspecting chemical/ammonia burn. I have also put in a dose of Pondzyme Barley. The ammonia was HIGH 2ppm just after this bog was discovered. Likely from the two dead fish in there for a few hours before discovery. Since the pond is not cycled, there would be nothing to check that ammonia rise at all.

I think I have deduced what may have happened. My pond inhabitants are notorious for "deadly mating". Twice this year I have had fish kill from an insane unexpected mating frenzy. The largest fish, a butterfly koi, was indeed female. When found she was near disintegrating, and totally beaten up. There is no evidence of an animal at the pond, not even water slpashed over the wood sides. The other dead koi, was a juvenile, about 3 inches or so in length, also female. They have mated before, and were so banged up after it, the damage appears the same, only just a bit worse. I suspect the heat triggered mating, and these two just got beaten up and chased down to death. Without me actually home monitoring (to separate them out), they were killed, and left in that heat and unchecked, their decomposition poisoned the pond.

This scenario has happened before with one other Koi dying after unchecked mating, and my prized large Sarsa Comet early in the spring. The difference is these fish were much smaller, and were found quickly thereafter.

I can't be sure this is what happened, but its the only explanation that makes any sense.